Fluorescence detection is widely used to detect many types of chemical and biological agents. Typically, fluorescence emission spectra are measured from fluorescence emitting entities, such as microorganisms and chemical compounds, by way of example. Conventional fluorescence detection is very inefficient, however, because in prior art fluorescence detection systems the vast majority of the fluorescence remains uncollected and is wasted.
A prior art fluorescence detection system typically includes a source of fluorescence excitation light, such as a laser, and a sample containing one or more fluorescence emitting agents. The excitation light is directed to the sample, and induces any fluorescence emitting agent present in the sample to fluoresce. An optical detector monitors the fluorescent light emitted by the agent. Typically, a fiber optic waveguide is used to guide the return light from the sample to the sensor. As known in the art, fiber optic waveguides depends on total internal reflections to confine and guide incident light within the fiber optic core.
In prior art fluorescence detection systems, most of the fluorescence does not fall onto the optical detector, and therefore is not collected. When highly sensitive tests are required, this lost fluorescence leads to an undesirable increase in the minimum detectability limits. Also, excitation light and any other background light must be separated from the desired fluorescence signal. In many situations, the signal-to-noise ratio may not be sufficient to prevent the lost signal and the background noise from causing a significant problem.
Further, conventional fiber-optic waveguides require the index of refraction of the cladding to be lower than the index of refraction of the fiber optic core, where the fluorescence takes place. It is very difficult to find structural cladding material with suitable indices of refraction, however, because fluorescence detection of chemical and biological agents is commonly done in a solution. The typical glasses and plastics, from which the claddings of optical fibers are made, have refractive indices that are significantly higher, as compared to the refractive index of aqueous solutions. It is therefore hard to provide a cladding material with suitable indices of refraction, especially when fluorescence detection is performed in an aqueous solution.
While various techniques have been implemented in the prior art to increase the index of the aqueous solution, or to decrease the index of refraction of the confining material, such techniques have only been applicable to a relatively small number of situations. In most situations, the fluorescent molecules were either attached to the outside of the optical fiber core, or were suspended in the cladding of the fiber. In both cases, the collection efficiency of the system is greatly reduced.
It is an object of the present invention to overcome the above-described limitations of prior art fluorescence detection systems. It is another object of the present invention to greatly increase the sensitivity of fluorescence based chemical and biological detectors.
Recently, photonic band gap structures have received a lot of interest from researchers. Unlike optical fibers, photonic band gap structures allow light within certain well-defined wavelength bands to be guided without a total internal reflection mechanism. Photonic band gap structures are configured so as to confine and guide light through resonant reflections, and do not depend on total internal reflections. Accordingly, much greater flexibility is allowed in the design and construction of such structures. For example, the core of a photonic band gap structure is not restricted to materials having a higher index of refraction, as compared to the cladding of the photonic band gap fiber.
It is another object of the present invention to use photonic band gap fibers to significantly increase the sensitivity and selectivity of fluorescence detection systems.